1. Field of the Invention
The present invention relates to a stator core of a vehicle rotary electric machine, such as an AC generator, to be mounted in a passenger car, a truck, etc.
2. Description of the Related Art
In order to supply an inexpensive vehicle rotary machine, a magnetic stator core is usually comprised of a spirally wound magnetic strip. Such a stator core can provide a high yield percentage magnetic material and can be manufactured at a high manufacturing speed.
In order to meet a recent demand for a high power generator, conductors are to be disposed in the slots of the stator core at as a high space factor as possible.
As shown in JP-A11-299136, a pair long and thin magnetic sheets is stamped out to form a pair of magnetic strips that has a plurality of teeth and a core back. In order to increase the yield percentage of the magnetic strips, the teeth of one of the pair of magnetic strips are respectively formed from portions inside slots of the other magnetic strip. Thereafter, each the magnetic strip is spirally wound into a cylindrical stack with the slots of one layer being aligned with those of another layer until the cylindrical stack has a prescribed thickness. Then, the stack is welded or riveted. Because two or more magnetic strips are formed at one time, the manpower and cost for manufacturing the stator cores can be reduced.
In order to moderate stress of a magnetic strip when it is spirally wound, JP-A-48-24204 discloses teeth whose tooth width becomes narrower toward the core back. In other words, slot width is widened at the middle thereof. This reduces strain of the strip when it is spirally wound and makes the size and shape of a cylindrical stator core accurate.
Because the magnetic reluctance of the air gap between the rotor and the stator is the largest in the magnetic circuit of a generator, the tooth edge formed at the inner periphery of the stator has a wider cross-section of a magnetic path than the rest of the tooth. In other words, each tooth has a pair of circumferential projections to increase the width of the tooth.
As the space factor of the conductors increases, the slot area is narrowed to thereby widen the cross-section of the tooth. If the teeth of one of the pair of magnetic strips are respectively formed from portions inside slots of the other magnetic strip as disclosed in JP-A-299136, it is not possible to provide such a circumferentially wide tooth.
If the tooth width is narrowed at the middle thereof as shown in JP-A-48-24204, a widened tooth edge of one of a pair of magnetic strips can be formed from the portion inside a widened slot of the other magnetic strip. However, if the tooth is narrowed at the middle, the magnetic reluctance of the teeth increases, and the magnetic flux flowing through the stator core decreases significantly. This lowers the output power.
A stator core of a vehicle AC generator that has an outside diameter between 90 mm and 140 mm is usually comprised of a laminated steel strip of a thickness between 0.5 mm and 1.0 mm. It is necessary to have a margin (or clearance) if teeth and slots are formed from a steel strip of the above thickness.
However, if the thickness is reduced to a thickness less than 0.2 mm, the edge of the sheet may be drawn because of the low rigidity thereof. As a result, the projections of the tooth edge can not have proper length, and a sufficient cross-section of the magnetic flux from the rotor can not be provided.